Monday, September 28, 2015

Early days - tether systems (4/4)

Fourth of four. My thinking has varied over time and I'm currently favoring Spectra/Dyneema fiber vs. Zylon. Regardless, this is a broad outline for what would be a technically challenging program.

This post is incomplete and does not contain financial estimates as do the other three, partly because actual tethers assembled in the context of a comprehensive campaign would have to consider the availability of mass from captured asteroids or sling-launched from the lunar surface. I put a couple of months into the problem and have not yet arrived at solid numbers.

Nonrocket Propulsion Program

This program aims to construct momentum exchange tether transportation systems in low Earth orbit, Lunar orbit and attached to Phobos. Payloads will be transferrable from low-Earth orbit to the Lunar surface with dV requirements as low as 25 m/s. Lunar fuel and products can be hooked directly from the surface and launched to LEO with a similar dV for course correction. The Phobos tether will initially have similar uses transferring from Phobos to Deimos or low Mars orbit with minimal propellant, but the full tether will be able to deploy cargo to Ceres and other asteroid belt targets or back to Earth orbit, perform capture of interplanetary craft from Earth as well as carry suborbital vehicles to and from the Martian surface.

The LEO tether system masses 20.4t (23.9t including the upper stage counterweight). Using a Falcon 9H instead of a Delta IVh would substantially increase the system’s payload and throughput limits. Payloads of 1-2t could be launched to LTO every month. This system will require ion thruster fuel periodically; using a Falcon 9H would allow for up to 32.6t of cargo (a year’s supply of fuel and payloads) along with the initial launch.

The Lunar tether system masses 16.7t, which is just over the capacity of a Falcon 9h. The empty upper stage is estimated to mass 3.9t and can be used as part of the counterweight mass, so this launch vehicle is still preferred.

Reliable cost estimates are not available, so we will assume roughly $500m development and $200m for each spacecraft in the LEO-Luna system. The full program will also require a Lunar lander and ISRU / sample collection equipment to handle material delivery to the tether point.

The Phobos tethers will be a longer-term development program somewhat smaller in scale than a full Lunar elevator. Initial phases would allow for fuel savings when transferring between Mars orbits, Phobos and Deimos. Later phases would carry Mars landers to the surface for little or no fuel and capture shuttles from the surface for about 500m/s dV on the Marsward tether. The outer tether would handle payloads to or from trans-Earth or trans-Ceres orbits without consuming fuel. Phobos is an immense momentum bank, so this system has the advantage of not requiring fuel to re-boost the tethers.

This system requires two tethers. The Spaceward tether would be deployed first and used to capture payloads from Earth. This tether is Zylon, 6155km long and masses {note: not settled, requires further research}. The Marsward tether is Zylon, 5,770km long, taper ratio 8:1 and masses 11.23 times the design payload in its most basic configuration. This requires self-powered climbers. Each Falcon 9H flight would deliver enough material for 1 ton of payload capacity. At 15m/s a payload can be handled every 4.5 days; the design can handle its own mass in about 50 days. For a 15-ton lander about 170t of tether needs to be deployed, requiring at least 15 launches (margin of 2t for structure and navigation).

Extended Lunar program development is aimed at a full L1 or L2 Lunar space elevator and surface tramway connecting the equatorial anchor site to one pole. This program will require an additional 8,000t of material; if all of it is shipped from Earth that would mean 160 launches or about $24 billion over 14 years. Use of captured asteroid material and/or Lunar surface material as counterweights, solar panels and structural members could reduce this by perhaps 40%, while use of locally-produced tether materials from hydroponic sources could eliminate the remaining ~60%. Trades will require examination. A commitment to this program would provide a means of valuing the products of other programs in terms of the savings earned against this one.